Microwave heating apparatus



Filed March 20, 1967 FIG. 4

PRIOR ART INVENTOR. REXFORD E. BLACK MLz XXB ATTORNEY Sept. 1, 1970 2 'J"""""' G F F B I U w I U I 2 H .l 20 I 5 q G mm H m 7 2 F W B M 6 7 5 W m 5 R47 A T W M R F. Plv mm M G United States Patent fornia Filed Mar. 20, 1967, Ser. No. 624,503 Int. Cl. H05b 9/06 US. Cl. 219-1055 8 Claims ABSTRACT OF THE DISCLOSURE The microwave heating apparatus has a rectangular microwave oven which acts as a multimode cavity resonator. The oven is provided with a door hinged at the front thereof. The oven is excited by coupling a microwave energy source to a waveguide feed centrally located in the top wall of the oven. A disc-shaped conductive member whose diameter is greater than one-half the free space wavelength of the applied energy is rotably mounted at its center to the back wall of the oven. Diametrically opposite equal segments of the disc are bent to incline towards the back wall of the oven.

BACKGROUND OF INVENTION The present invention relates to microwave heating apparatus, and more particularly, to a microwave heating apparatus in which the mode pattern of the heating electromagnetic field is changed to produce a more uniform electromagnetic field distribution in the working region.

In heating materials with microwave energy, it is the practice to expose the work piece to an electromagnetic field established in an excited microwave resonating device. To accomplish uniform heating of the work piece, especially one having a dimension on the order of the free space wavelength, A, of the applied energy, generally, the microwave resonant device is constructed so as to support a multiplicity of field intensity distributions, or mode patterns. Such a device is commonly referred to as a multimode microwave resonator. By causing the field intensity distribution to be periodically changed, i.e., mode stirring, the total heat energy available to all portions of the microwave resonator is made more uniform thereby effecting a uniform heating of the work piece.

Heretofore, various mechanical and electronic techniques have been employed to effect the periodic chang ing of field intensity distribution. Electronic mode stirring techniques involve either modulating the frequency of the microwave energy source or providing multiple inputs to the microwave resonator. Such electronic techniques involve either complicated microwave energy sources or critical placement of the input waveguide feed relative to the microwave resonator.

With respect to mechanical mode stirrers, basically there are two types; moving antenna feeds, and those which effect a change in the geometric space of the microwave resonator as seen by electromagnetic field established therein. For practical reasons, of the two types, the moving antenna feed has proven the least suitable. This is because the Waveguide plumbing feeding the moving antenna requires an exceedingly complex construction in order to prevent reflected damaging microwave energy from reaching the microwave energy source.

Changes in the geometric space seen by the electro- 3,526,737 Patented Sept. 1, 1970 magnetic field has been accomplished a number of ways. For example, microwave resonators have been constructed with deformable walls which when moved effect a change in the geometric space. Such devices have the disadvantages of being structurally complex, requiring complex prime movers for the deformable wall, not being able to accomplish rapid shifting of the field distribution in the microwave resonator, and causing microwave energy reflections which are detected by the power meter monitoring the output of the microwave energy source as variations in the output power of the source.

Reciprocating and revolving conductive members translatably mounted within the microwave resonant device also have been employed to effect a variation in the geometric space as seen by the electromagnetic field. Both have certain advantages over the deformable walltype mode stirrer. However, the revolving conductive member mode stirrers are superior to the reciprocating types in that they can accomplish a considerably more rapid shifting of the field distribution and they require less complex prime movers. Unfortunately, however, the structures of prior art revolving type mode stirrers have undesirable features which if eliminated would substantially enhance their value. Most importantly, prior art revolving type mode stirrers generally include a plurality of projecting members, such as vanes. Such structures are hazardous in that when in motion, they can cause serious injury to anyone contacting same. Furthermore, vaned structures and the like are more diflicult to clean and maintain than, for example, the uniform surface of some of the reciprocating type mode stirrers. In addition, when in motion, the vaned mode stirrers cause air currents to be established within the resonant device. Such air currents often are undesirable especially, for example, when the weight of the work piece is monitored precisely within the resonant device while being heated.

SUMMARY OF THE INVENTION The present invention is a microwave heating apparatus which overcomes the limitations and disadvantages of the prior art devices. More particularly, the microwave heating apparatus of the present invention includes a mutimode microwave resonant device adapted to be excited by a microwave energy source. Pursuant to the present invention, to effect changing of the electromagnetic field distribution established within the excited resonant device, solid disc-shaped sheet of material reflective of electromagnetic fields having a continuous surface is rotatably mounted within the resonant device. The sheet has a central portion and two identical segments angularly extending therefrom. In addition to the advantages accruing to the present invention because the sheet is rotatably mounted, since the sheet has a continuous surface, it can be easily cleaned. Ease of cleaning is particularly important in those instances where the microwave heating device is used to treat work pieces which tend to spatter material during the heating process, for example, as in the case of ovens used for cooking food stuffs. Furthermore, when in motion, the rotatably mounted body presents a continuous surface to any object which it is likely to encounter. Consequently, the hazards normally associated with the prior art revolving type mode stirrers are markedly reduced.

Accordingly, it is an object of the present invention to provide a microwave heating device for uniformly heating a work piece.

More particularly, it is an object of the present invention to provide a microwave heating device which accomplishes uniform heating of a work piece by mechanically varying the electromagnetic field distribution in a multimode cavity resonator with a revolving type mode stirrer which minimizes the hazards to personnel.

It is a further object of the present invention to provide a microwave heating device including a revolving type mode stirrer which can be conveniently cleaned.

Another object of the present invention is to provide a microwave heating device including a revolving type mode stirrer which is both statically and dynamically stable.

It is still a further object of the present invention to provide a microwave heating apparatus suitable for uniformly heating work pieces having dimensions which are on the order of the free space wavelength of the applied energy.

BRIEF DESCRIPTION OF DRAWING The foregoing and other objects and advantages of the microwave heating apparatus of the present invention will become more apparent from the following detailed description and appended claims considered together with the accompanying drawing in which:

FIG. 1 is a front elevation view of one embodiment of the microwave heating apparatus of the present invention.

FIG. 2 is a front elevation cross section view of the microwave heating apparatus of FIG. 1.

FIG. 3 is an enlarged end cross section view of the mode stirrer taken along lines 3-3 of FIG. 2.

FIG. 4 is an end cross section view of a prior art mode stirrer.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. 1-3, the microwave heating apparatus of the present invention includes a multirnode microwave resonant device 11 of aluminum or other conductive materials. The resonant device 11 defines a compartment 12 for receiving a work piece to be heated by microwave energy. The size and configuration of the compartment 12 are selected so that a large number of different electromagnetic field distribution patterns can be established therein. To effect heating of a work piece, the microwave resonant device 11 is coupled at input waveguide feed 13 to a microwave generator 14 by a waveguide 16. Microwave generator 14 is operated to provide suitable high frequency as, for example, 2450 megacyeles (me), microwave energy to excite the resonant device 11.

To vary the electromagnetic field distribution established within compartment 12 and thereby effect a more uniform heating of the work piece therein, a mode stirrer 17 is rotatably mounted within compartment 12. In accordance with the present invention, the mode stirrer 17 is a body of material reflective of electromagnetic fields, for example, a conductive material such as aluminum, defining a disc-shaped sheet having a continuous surface 18. The mode stirrer 17 has a central portion 23 and at least two segments 24 and 26 extending angularly therefrom to one side of the plane of the central portion. As the mode stirrer 17 is rotated, the electromagnetic field established therein sees a change in the geometric space of the compartment 12. As a result of this variation in the geometric space of the compartment 12, the electromagnetic field distribution will be changed accordingly to assume various mode patterns. By changing the mode pattern, a more uniform distribution of the electromagnetic energy is obtained within compartment 12. The more uniform the energy distribution, the more uniform is the heating of a work piece. Substantially uniform electromagnetic energy distribution can be realized by causing a large number of mode pattern changes. Pursuant to this end, the

mode stirrer 17 would be constructed so that the dimensions of the portion 19 of the continuous surface 18 are at least M 2.

Although in accordance with the present invention, mode stirrer 27 can take the form of various configurations, the embodiment illustrated in FIGS. 2 and 3 has been found to be particularly suited for the practice of the present invention. The mode stirrer 17 of FIGS. 2 and 3 is a solid circular disc-shaped body of inch thick aluminum sheet having a central portion 23 and at least two segments 24 and 26. To facilitate revolving the discshaped mode stirrer 17 under conditions of dynamic stability, the mode stirrer 17 is constructed to consist of at least two identical sectors, each including one of the segments 24 and 26. In the two sectored configurations of FIGS. 2 and 3, segments 24 and 26 are located diametrically opposite. The segments 24 and 25 are bent respectively at 27 and 28 toward the wall 21 of the microwave resonant device 11 so that the surfaces 19 defined thereby proximate the wall 21 each are at an angle 0 relative to the wall 21.

Preferably, the mode stirrer 17 is rotatably mounted with its center coinciding with the axis of revolution 22 and its central portion 23 perpendicular to the axis 22. In this way, the mode stirrer 17 will be dynamically stable thereby greatly simplifying the rotatable mounting thereof.

The mode stirrer 17 is rotated by a drive motor 29 mounted exteriorly of the microwave resonant device 11. Rotating motion is coupled from the motor 29 to the mode stirrer 17 via a drive shaft 31 extending through an aperture 32 of the wall 21. A T-shaped hub assembly 33 is secured to the central portion 23 of the disc-shaped mode stirrer 17 by screws 34 extending through the central portion 23 to threadingly engage hub 33. The stem 36 of the hub assembly 33 defines a receptacle 37 for receiving the drive shaft 31. The drive shaft 31 is fastened within the receptacle 37 by set screws 38. To provide support for the long drive shaft 31, a journal box 39 housing axially spaced bearings 41 is provided to journally support the drive shaft 31. The journal box 39 is fastened in place by screws 42 threadingly engaging an aluminum support plate 43 secured, as by heliarc welding, to the wall 21.

In providing an aperture 32 in wall 21 so that the drive shaft 31 can extend exteriorly of the resonant device 11, electromagnetic energy is allowed to escape from the resonant device and to be lost to the surroundings. In those instances where heating is conducted at high power levels, such escaping energy often is hazardous. To prevent the escape of such energy, a one-quarter wavelength shorting stub 44 is provided. Specifically, the shorting stub 44 is a T-shaped conductive member having a bar section 46 and stem section 47. The bar 46 is fastened to the inside of wall 21 by screws 48 threadingly engaging the support plate 43 through the wall 21. The stem 47 extends from the bar 46 to the interior of the resonant device 11, and together with the bar 46 define a passageway 49 to pass the drive shaft 31. The length of the stem 47 is adjusted to N4 of the applied energy thereby preventing the propagation of electromagnetic energy through passageway 49 and hence aperture 32 to the exterior of the resonant device 11.

As the mode stirrer 17 is rotated by drive motor 29, the electromagnetic field mode pattern is cycled through a sequence of different mode patterns. The number of cycles of the mode pattern sequence per revolution of the mode stirrer 17 is equal to the number of repeating congruent orienations relative to the wall 21 taken by the mode stirrer 17 during a single revolution. For a mode stirrer constructed of a plurality of identical sectors, such as the sectors of the embodiment of FIGS. 2 and 3, the number of cycles of the mode Pattern sequence per revolution is equal to the number of identical sectors composing the mode stirrer 17. Hence, the rapidity of change in the electromagnetic field distribution is influenced by both the speed of revolution of the mode stirrer 17 and the number of repeating congruent orientations taken by the mode stirrer 17 during a single revolution. However, to insure that a large number of mode pattern changes occur, hence, a substantially uniform electromagnetic energy distribution within compartment 12, the dimensions of the sectors should be at least )\/2 of the applied energy. Therefore, to realize the substantially uniform electromagnetic energy distribution, a mode stirrer consisting of a large number of identical sectors probably would have to be larger, hence more massive, than one consisting of fewer identical sectors of the same configuration.

FIG. 4 illustrates a prior art mode stirrer. However, contrary to the dynamically stable mode stirrer embodiment of FIGS. 2 and 3, the prior art mode stirrer of FIG. 4 dynamically unstable. As noted hereinbefore, such prior mode stirrer presents complications in mounting it for rotation.

From the foregoing description it is seen that because the mode stirrer of the invention has a continuous surface, it is considerably easier to clean than most prior art revolving-type mode stirrers. With respect to the safety feature of the revolving-type mode stirrer of the present invention, it is seen that any object encountering the mode stirrer will tend to meet a continuous surface instead of an edge transversely moving with respect to the object, for example, as would be the case with the prior art fan-type mode stirrers.

It should be appreciated that other mode stirrer embodiments are possible within the contex of the present invention than the above specifically described and illustrated embodiments. Examples of some of these other mode stirrer embodiments are ellipsoids; elliptic paraboloids; conic sections, regular paraboloids and partial spheroids; wedges; polygonal bodies; and non-circular sheets.

Referring again to FIGS. 1-3, the mode stirrer 17 is shown as being mounted within a completely closed multimode resonant device with perpendicularly intersecting walls for batch heating work pieces placed therein. However, the resonant device 11 could be adapted to heat work pieces continuously transported therethrough by providing suitable inlet and outlet ports to allow conveying of the work pieces through the resonant device 11. Alternatively, the multimode resonant device could be employed to excite another resonant device in which the work pieces are located for heating. Furthermore, resonant devices of configurations other than those having perpendicularly intersecting walls can be combined with the mode stirrer 17 to accomplish the objectives of the present invention.

The resonant device embodiment of the figures is an oven for heating work pieces. As shown therein, the resonant device 11 is rectangular shaped having a front opening door 53 hinged at 54. When closed, the door 53 snugly seats in the opening to the compartment 12. The door is held closed by a pivotally mounted bar 56 and bar receiving member 57. To minimize the effects of the placement of the mode stirrer 17 on the number of mode patterns formed by revolving the mode stirrer 17, the resonant device 11 is symmetrically loaded by placing the input waveguide feed 13 centrally in the top wall 58 of the resonant device 11. Viewing to the interior of the compartment is provided by a view port 59 cut into the door 53. To prevent the escape of electromagnetic energy through the view port 59, a conductive mesh 61 is disposed in covering relation to the view port 59. The cross sectional dimensions of the apertures 62 defined by the mesh 61 are adjusted to be less than M, the cutoff wavelength of the applied electromagnetic energy.

One embodiment of the microwave heating device of the present invention constructed in accordance with the embodiment illustrated in FIGS. 1-3 for operation at a microwave frequency of 2450 mc. had the following specifications. The compartment 12 was 24 inches wide, 18 inches high and 24 inches deep. The disc-shaped mode stirrer 17 had a diameter of 7 inches in its unbent configuration and a central portion 1 /2 inches wide. The segments 24 and 26 were bent to define an angle 0 of 30. With an input power of 2500 watts and the mode stirrer 17 revolved at 600 r.p.m., the microwave heating device was operated to dry cement blocks having a volume of 274 cm. and a water content of 5% by volume. The water content was reduced to 1.7% in minutes. This cure time is much shorter than that necessary by standard convection hot air ovens, i.e., a normal 24 day cure cycle at 200 F.

Although the present invention has been described in detail with reference to particular embodiments, from the description it is apparent that many modifications and variations are possible without departing from the scope of the invention. Hence the present invention is not intended to be limited except by the terms of the following claims.

What is claimed is:

1. A microwave heating apparatus comprising a multimode microwave resonant device having walls defining a compartment and adapted to be excited by a microwave energy source; and a solid sheet of reflective material mounted within said compartment proximate a wall of said resonant device for rotation about an axis passing through the center of said sheet, said sheet having a central portion containing said center and at least two opposed segments extending angularly from the plane of said central portion to one and the same side of said plane.

2. The apparatus according to claim 1 wherein said microwave resonant device has a waveguide feed joined to a wall thereof for coupling microwave energy from said source to excit said compartment, said waveguide feed located at the center of said wall.

3. The apparatus according ai claim 1 further comprising a drive motor for rotating said sheet located exteriorly of said resonant device, a drive shaft for coupling rotating drive from said motor to said sheet operatively coupled to said drive motor and passing to the interior of said resonant device through an aperture defined by a wall of said resonant device, hub means for securing said sheet to said drive shaft to be rotated by said drive shaft, and a tubular member surrounding said drive shaft spaced therefrom and defining a passageway in registry with said wall aperture, said tubular member extending from said wall to the interior of the resonant device a distance of 4 where A is the free space wavelength of the microwave energy provided by said source.

4. The apparatus according to claim 1 wherein said microwave resonant device includes a plurality of walls joined to define a rectangular oven compartment, one of said walls defined by a hinged door for placing work pieces within said compartment, said sheet is rotatably mounted within said oven at a first fixed wall to be driven by a motor located exteriorly of said oven, and further including a waveguide feed joined to a second fixed wall at the center thereof for coupling microwave energy from said source to excite said oven compartment.

5. The apparatus according to claim 1 wherein each of said segments has a dimension in the direction away from said central portion at least A/ 2, where A is the free space wavelength of the microwave energy provided by said source.

'6. The apparatus according to claim 1 wherein said sheet has circular plan view, and said segments are chord segments having radial extents of at least 2 and angularly extending from diametrically opposite sides of said sheet.

7. The apparatus according to claim 1 wherein said microwave resonant device includes at least one wall having a flat inner surface, and wherein said sheet is rotatably 7 8 mounted at said flat surfaced wall to have its central 3,308,261 3/1967 Velander 219-10.55 portion generally parallel thereto. 2,583,338 1/1952 Morse et al 219101.55 8. The apparatus of claim 7 wherein said two opposed 2,961,520 11/ 1960 Long 219-10 .55 segments extend angularly from the plane of said central 3,182,166 5/ 1965 Bohm et al. 219-1055 portion toward said one wall of said resonant device. 5 3,431,381 3/ 1969 Anderson 21910.55

References Cited OTHER REFERENCES UNITED STATES PATENTS .zsFrlitgzzGern-ran application No. 1,132,268 printed June 2,813,185 11/-l957 Smith 21910.55 I 2,9 7 1/1960 Haagensen 21910-55 JOSEPH v. TRUHE, Primary Examiner 3,321,605 5/1967 Reker 219-1055 3,364,332 1/1968 Reftmark 219-10.55 BENDER Assistant Examiner 

